Selenium is an essential element in mammals and birds, and its remarkable biological effects can be related to unique functions of various selenoproteins. Discovery of a new selenoprotein will provide a significant insight into undiscovered biological functions of selenium in human health. This study reports isolation and characterization of a new selenoprotein from human lung tumor cell NCI-H441, which was grown in RPMI-1640 medium containing 10% fetal bovine serum and 100 nM 75Se-sodium selenite. A 75Se-labeled protein was isolated from the sonic extract of the tumor cells by DE-23, phenyl-Sepharose, heparin-Agarose and butyl-Sepharose column chromatography. The purified protein is a homodimer of identical subunits of 57,000, and it was shown to have selenium as selenocysteine. The selenoprotein does not have detectable amounts of glycosyl groups, and it did not react with anti-rat selenoprotein P antibody. The selenoprotein contains FAD as a prosthetic group, and shows thioredoxin (Trx) reductase activity based on NADPH-dependent reduction of 5,5'- dithiobis(2-nitrobenzoic acid), and Trx-insulin coupled assay.